Engine error detection system

ABSTRACT

A variety of methods and arrangements for detecting misfire and other engine-related errors are described. In one aspect, a window is assigned to a target firing opportunity for a target working chamber. There is an attempt to fire a target working chamber during the target firing opportunity. A change in an engine parameter (e.g., crankshaft angular acceleration) is measured during the window. A model (e.g., a pressure model) is used to help determine an expected change in the engine parameter during the target firing opportunity. Based on a comparison of the expected change and the measured change in the engine parameter, a determination is made as to whether an engine error (e.g., misfire) has occurred.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Patent Application No.62/064,786, entitled “Misfire Detection in a Dynamic Skip Fire Engine,”filed Oct. 16, 2014; and U.S. Patent Application No. 62/148,636,entitled “Engine Error Detection System,” filed Apr. 16, 2015, each ofwhich is incorporated herein in its entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to powertrain diagnostics.Various embodiments pertain to a misfire detection system and anadaptive model for detecting engine- or working chamber-related errors

BACKGROUND

Skip fire engine control is understood to offer a number of benefitsincluding the potential of increased fuel efficiency. In general, skipfire engine control contemplates selectively skipping the firing ofcertain cylinders during selected firing opportunities. Thus, forexample, a particular cylinder may be fired during one firingopportunity and then may be skipped during the next firing opportunityand then selectively skipped or fired during the next. This iscontrasted with conventional variable displacement engine operation inwhich a fixed set of the cylinders are deactivated during certainlow-load operating conditions.

In this manner, even finer control of the effective engine displacementis possible. For example, firing every third cylinder in a 4 cylinderengine would provide an effective displacement of ⅓^(rd) of the fullengine displacement, which is a fractional displacement that is notobtainable by simply deactivating a set of cylinders. Similarly, firingevery other cylinder in a 3 cylinder engine would provide an effectivedisplacement of ½, which is a fractional displacement that is notobtainable by simply deactivating a set of cylinders. U.S. Pat. No.8,131,445 (which was filed by the assignee of the present applicationand is incorporated herein by reference in its entirety for allpurposes) teaches a variety of skip fire engine control implementations.

When a cylinder is deactivated in a variable displacement engine, itsvalves are not actuated and although the piston typically stillreciprocates, fuel is not combusted during the power stroke. Since thecylinders that are “shut down” don't deliver any net positive torque,the proportionate load on the remaining cylinders is increased, therebyallowing the remaining cylinders to operate at an improved thermodynamicefficiency. With skip fire control, cylinders are also preferablydeactivated during skipped working cycles in the sense that air is notpumped through the cylinder and no fuel is delivered and/or combustedduring skipped working cycles when such valve deactivation mechanism isavailable. Often, no air is introduced to the deactivated cylindersduring the skipped working cycles thereby reducing pumping losses.However, in other circumstances it may be desirable to trap exhaustgases within a deactivated cylinder, or to introduce, but not releaseair from a deactivated cylinder during selected skipped working cycles.In such circumstances, the skipped cylinder may effectively act as a gasspring. Although deactivating skipped cylinders is generally preferred,it should be appreciated that in some engines or during some workingcycles it may not be possible, or in some situations desirable, to trulydeactivate cylinders. When a cylinder is skipped, but not deactivated,intake gases drawn from the intake manifold are effectively pumpedthrough the cylinder during the skipped working cycle.

Although the concept of skip fire control has been around for a longtime, it has not traditionally been used in commercially availableengines, so an additional challenge to implementing skip fire control isinsuring that the engine's other engine/power train systems workeffectively during skip fire control. One such system relates to enginediagnostics. As is well understood by those familiar with the art,modern vehicles incorporate engine management systems that performin-situ diagnostics on various powertrain and vehicle component duringvehicle operation. These diagnostic systems are often referred to as“On-Board Diagnostics” (OBD) systems and there are a number of enginediagnostic protocols that are performed while the engine is runningModern OBD systems store and report a significant amount of informationconcerning the operation and state of health of various vehiclesub-systems including the powertrain. For example, some OBD systems arearranged to detect a situation in which a cylinder misfires i.e., whenthe cylinder fails to fire or there is incomplete combustion in thecylinder.

Although prior art OBD systems are well suited to detect misfire in aconventional all-cylinder engine control system, they are generally illsuited for use in a skip fire engine control system. Various embodimentsof the present invention contemplate arrangements, methods andtechniques for detecting misfire in an engine operated in a skip firemanner.

SUMMARY

A variety of methods and arrangements for detecting misfire and otherengine-related errors are described. In one aspect, a window is assignedto a target firing opportunity for a target working chamber. In variousembodiments, the window is related to the rotation of the crankshaft.There is an attempt to fire a target working chamber during the targetfiring opportunity. A change in an engine parameter (e.g., crankshaftangular acceleration or another crankshaft-related parameter) ismeasured during the window. A model (e.g., a multi-cylinder pressuremodel) is used to help determine an expected change in the engineparameter during the target firing opportunity. In various embodiments,the engine is operated in a skip fire manner and the model takes intoaccount the skipping of one or more working chambers. Based on acomparison of the expected change and the measured change in the engineparameter, a determination is made as to whether an engine- or workingchamber-related error (e.g., misfire) has occurred. In variousembodiments, the model is adjusted dynamically based on the measuredchange in the engine parameter to help improve the accuracy of themodel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a flow diagram illustrating a method of detecting misfire in askip fire engine control system according to a particular embodiment ofthe present invention.

FIG. 2 is a block diagram of a misfire detection system according to aparticular embodiment of the present invention.

FIG. 3 is a block diagram of firing opportunities and associated angularwindow segments according to a particular embodiment of the presentinvention.

FIG. 4 is a block diagram of a misfire detection system according to aparticular embodiment of the present invention.

FIG. 5 is a graph of cylinder pressure as a function of crank angleaccording to a particular embodiment of the present invention.

FIG. 6 is a graph of cylinder pressure as a function of cylinder volumeaccording to a particular embodiment of the present invention.

FIG. 7 is a graph comparing a modeled cylinder pressure and a measuredcylinder pressure according to a particular embodiment of the presentinvention.

FIG. 8 is a graph of crankshaft acceleration as a function of timeaccording to a particular embodiment of the present invention.

FIG. 9 is a flow diagram illustrating a method of detecting an engineerror using an adaptive model according to a particular embodiment ofthe present invention.

FIG. 10 is a graph comparing modeled and measured crankshaftacceleration using a model according to a particular embodiment of thepresent invention.

FIG. 11 is a graph comparing modeled and measured crankshaftacceleration using an adaptive model according to a particularembodiment of the present invention.

FIG. 12 is a flow diagram illustrating a technique for using an adaptivemodel according to a particular embodiment of the present invention.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION

The present invention relates to systems for detecting engine- andworking chamber-related errors. More specifically, various embodimentsof the present invention relate to techniques and arrangements fordetecting misfire or other errors in skip fire engine control systems.

As noted in the background, prior art misfire detection systems aregenerally not suitable for detecting misfire in a dynamic skip fireengine control system. For example, various prior art misfire detectionsystems detect unexpected changes in the rotation speed of thecrankshaft and use this to determine if a misfire has occurred. Thisworks well in conventional, all cylinder engine operation, since it isexpected that crankshaft acceleration will remain generally consistent.Although there are some variations in crankshaft acceleration from onefiring to the next, the crankshaft acceleration peaks and profilesremain generally consistent in size and shape, in large part due to thefact that every cylinder is being fired. Thus, when a significantdeviation in crankshaft acceleration is detected with respect to thefiring of a particular cylinder, the misfire detection system maydetermine that the cylinder has misfired.

In dynamic skip fire engine operation, however, any working chamber orworking cycle may be skipped. That is, a particular working chambermight be fired during one working cycle, skipped during the next, andfired or skipped during the next. As a result, the crankshaftacceleration peaks and profiles may abruptly change as the firingsequence changes, even though all working chambers are properly firing,i.e. no misfires. Unlike in prior art misfire detection systems, anysubstantial drop in crankshaft acceleration cannot be assumed toindicate a misfire, since in a skip fire engine control system, selectedworking cycles may be skipped at almost any time, each of which may alsoresult in a drop in the crankshaft acceleration.

Conventional misfire determination systems also do not properly takeinto account the effect that the firing or skipping of various workingchambers have on a measurement of crankshaft acceleration in a skip fireengine control system. To illustrate this point, consider an example inwhich a designated cylinder is examined for a possible misfire.Combustion takes place in the designated cylinder during an assignedwindow (e.g., during at least part of the combustion stroke for thedesignated cylinder.) The crankshaft acceleration is measured duringthat window. If the crankshaft acceleration dips below a predeterminedthreshold, it is determined that a misfire has occurred in thedesignated cylinder.

In skip fire engine operation, the accuracy of the misfire determinationis improved if the misfire determination system and/or the misfirethreshold takes into account the impact of the skipping or firing ofother cylinders on the measured crankshaft acceleration. That is, thesystem should take into account the firing commands (i.e., skip or fire)for other cylinders that were executed prior to the window or will beexecuted after the window. It should be noted that while the firing ofthe designated cylinder may make the largest contribution to thecrankshaft acceleration during the window, there are a number of otherfactors that affect crankshaft torque. For example, it requires energyto compress the intake air during the compression stroke and that energycomes from the crankshaft thereby acting as a negative torque on thecrankshaft. Engines having multiple cylinders are generally designedwith their working cycles out of phase with one another at consistentintervals so that the compression of one cylinder occurs whilecombustion is occurring in another cylinder. In normal, all cylinderoperation, the torque generated by each firing, the torque required byeach compression stroke, and other engine generated torques tend to berelatively constant during steady state operation. Therefore, the evenspacing of the cylinder phases tend to result in each cylinder beingaffected in much the same way by events occurring in the othercylinders, which helps contribute to the consistency between the peaksand profiles associated with each firing opportunity during normalall-cylinder operation.

In skip fire operation, the effect of the other cylinders will notalways be so consistent. For example, in some implementations the valvesmay be operated in a manner in which the intake and exhaust valves areopened in the normal sequence during “fired” working cycles and are bothheld closed through skipped working cycles. This will result in thetorques applied to the crankshaft during each stroke of the workingcycle being different during a skipped working cycle than would be seenduring a fired working cycle. Most notably, during a skipped workingcycle in which low pressure exhaust trapping is used, only a smallamount of residual exhaust gases will remain in the cylinder andtherefore the torque imparted during the compression stroke in a skippedworking cycle will be quite different than the torque imparted duringactive (fired) working cycles because the relatively large negativetorques required for compression of the intake gases will be missingduring skipped working cycles. The trapped residual fraction can varybased on the valve timing and MAP influencing the imparted torque. Sincethe compression stroke associated with one cylinder will typicallyoverlap with the combustion stroke of another, the net torqueexperienced by the crankshaft during any particular combustion eventwill be affected by the firing decisions of other cylinders. Althoughthe compression stroke tends to have greatest impact, the differentialtorques experienced during the intake and exhaust strokes may also bedifferent in significant ways. For example, holding the intake valveclosed during the skipped working cycle may cause a very low pressure tobe developed in the cylinder during intake thereby imparting a largernegative torque during the intake stroke of a skipped working cycle thanwould occur during intake of an active (fired) working cycle.

Still further, different skip fire controllers may have different valveactuation schemes and/or may use a combination of different valveactuation schemes and such valve actuation schemes can further affectthe torque variations experienced by the crankshaft. For example, if anexhaust valve is not opened after a combustion event, a “high pressureexhaust gas spring” may effectively be created within the cylindercombustion chamber by the combustion gases and the timing of the exhaustvalve opening may be delayed from immediately after the combustion eventto a later working cycle. Such a high pressure spring will have asignificant impact on the torque applied during all of the otherstrokes. In another example particularly relevant to direct injectionengines, an intake valve may be opened in a working cycle in which nofueling or combustion occurs so that an air charge is trapped within thecombustion chamber during a skipped working cycle. Such events willaffect the net torque in yet another way. In still other circumstances,sometimes referred to as “re-exhaust”, it may be desirable to open theexhaust valve in the normal course after the firing of a cylinder andthen to reopen the exhaust valve in a subsequent skipped working cyclesuch as the one that immediately precedes an active (fired) workingcycle resulting in an extra exhaust valve opening event. In still otherimplementations, re-exhaust may be employed at the end of every skippedworking cycle. Of course, a variety of other valve actuation schemes maybe applied as well and it should be apparent that the timing andmagnitude of the torques applied to the crankshaft will depend on thestate of all of the cylinders.

This application contemplates various techniques for taking into accountat least some of the above factors in making a misfire determination ina skip fire engine control system. A particular embodiment contemplatesa misfire detection system that detects misfires using a multi-cylinderpressure model. The pressure model is used to model the pressure in someor all of the working chambers during a target firing opportunity. Invarious implementations, the pressure model takes into account whether aworking chamber is being fired or skipped, cylinder air charge due tocam phasing and intake manifold pressure, ignition timing adjustmentsand air fuel ratio variation. The pressure modeled for each workingchamber is then used to help determine whether a particular workingchamber is misfiring.

Referring initially to FIGS. 1 and 2, an example method 100 and system200 for detecting misfire using a multi-cylinder pressure model will bedescribed. FIG. 1 is a flow diagram illustrating the steps of the method100. The method 100 is implemented in the misfire detection system 200illustrated in FIG. 2. The misfire detection system 200 includes afiring timing determination module 202, a firing control unit 204, anengine parameter measurement module 206, a misfire detection module 208,and an engine 250. FIG. 2 illustrates an engine 250 having eightcylinders, labeled 1 through 8, as the working chambers. Although engine250 is shown having 8 cylinders arranged in two banks, engines havingdifferent numbers of cylinders arranged in different configurations maybe used. Also, although a variety of discrete modules are illustrated inFIG. 2, it should be appreciated that in various embodiments, themodules may be combined and/or operations of one module may instead behandled by another module.

Initially, at step 102 of FIG. 1, firing information is obtained by thefiring timing determination module 202 and/or the firing control unit204. The firing timing determination module 202 is arranged to issue asequence of firing commands used to operate the engine 250 in a skipfire manner and deliver a desired torque and/or firing fraction. Theskip fire firing sequence may be determined in a wide variety of ways.For example, the firing sequence may be generated using a sigma deltaconverter or any suitable control algorithm. In some embodiments, thefiring sequence is selected from a library of predefined firingsequences. The sequence of firing commands is transferred to the firingcontrol unit 204.

The firing control unit 204 is arranged to orchestrate the firings ofthe working chambers of the engine 250 using the received firingsequence. The firing control unit 204 receives data identifying suitableworking chambers from any suitable source (e.g., the engine 250) andmatches a selected working chamber to each firing command. Consider asimple example in which the firing control unit receives a short firingsequence of 0-1-0-0 from the firing timing determination unit 202, whichindicates a skip, fire, skip and a skip, respectively. In this example,the engine may be configured so that the cylinder firing opportunitiesare arranged in a repeating sequence of 1-8-7-2-6-5-4-3. That is, thefirst cylinder to have a firing opportunity may be cylinder 1, followedby cylinder 8, and then cylinder 7, etc. The firing control unit 204determines which cylinders should be matched to each firing command(e.g., it may determine that cylinders 1, 8, 7 and 2 should be skipped,fired, skipped and skipped, respectively, in accordance with thesequence.) Various embodiments of the present invention contemplatesusing such firing information (i.e., the firing sequence and theidentities or numbers of the corresponding working chambers) to helpdetect misfires. Note that the fire/skip information is typicallyavailable before execution of a firing/skip command, since time isneeded to fuel the cylinder and activate/deactivate the valves.

At step 104 of FIG. 1, the engine parameter measurement module 206assigns windows to each firing opportunity. The window may be anysuitable time period or interval that corresponds to a target firingopportunity of a target working chamber. A particular engine parameterwill later be measured across the window to help determine if a misfirehas occurred. The characteristics of the window may differ depending onthe type of engine parameter measurement.

In one embodiment, for example, the engine parameter to be measured iscrankshaft angular acceleration. The crankshaft angular accelerationtends to increase when combustion occurs in the target working chamber.As a result, a suitable window may be one that covers at least part ofthe power stroke for the target working chamber.

In another embodiment, the engine parameter to be measured is acombustion exhaust gas property. That is, one or more sensors in theexhaust system detect levels of oxygen or other components in an exhaustgas “pulse” that is generated during the firing opportunity. Thisanalysis is used to help determine whether a misfire has occurred. Thismeasurement may occur over a different window. Since exhaust gases areinvolved, the appropriate window may cover or correspond to at least aportion of the exhaust stroke of the target working chamber.Additionally, the window may also incorporate an offset to account forthe time needed for the corresponding exhaust “pulse” to traverse fromthe exhaust valve to the exhaust sensor. Generally, the window may varywidely, depending on the characteristics of the misfire detection system200. The exhaust sensor method of sensing misfires may be combined withthe crankshaft acceleration method and other possible means of misfiredetection.

An example of an association between windows and firing opportunitiesfor a corresponding working chamber is illustrated in FIG. 3. In thisexample, a total of 270° of rotation for the crankshaft of an eightcylinder, four-stroke engine is shown. During the rotation, there arethree firing opportunities, corresponding to the firing or skipping ofworking chambers 1, 8 and 7. A window is assigned to each of the firingopportunities and working chambers. Each window is an angular windowsegment that corresponds to a 90° rotation of the crankshaft. FIG. 3illustrates an example angular window segment 302, which corresponds toa firing opportunity for working chamber 8. The angular window segment302 begins at or around the time the piston for the correspondingworking chamber reaches top dead center (TDC) (e.g., at the beginning ofa power stroke in a four stroke engine.) It should be appreciated thatthe above example is used for illustrative purposes and that thecharacteristics of the windows and the way in which they are assignedmay vary for different applications. For example, it should beappreciated that windows longer or shorter than 90° rotation of thecrankshaft rotation may used. The length of the window may vary with thenumber of cylinders in the engine. For example, longer windows may beused in engines with fewer cylinders, since there are fewer firingopportunities per engine revolution. Also, the time windows associatedwith each cylinder may overlap.

Returning to the flow diagram of FIG. 2, the engine parametermeasurement module 206 measures a change in an engine parameter duringthe corresponding window (step 106 of FIG. 1) This measurement may beobtained, for example, using one or more sensors (e.g., a crankshaftposition sensor, an exhaust gas sensor, etc.) The engine parametermeasurement module 206 receives any input or engine parameter needed toperform the measurement, e.g. engine speed data, cylinder identityinformation, firing information from the firing timing determinationmodule 202/firing control unit 204, etc. A variety of different engineparameters may be measured during the window. In some embodiments, forexample, a crankshaft-related parameter or crankshaft angularacceleration is measured.

Below is one example formula for calculating crankshaft angularacceleration for the angular window segment 302 of working chamber 8 asshown in FIG. 3. In FIG. 3, the angular window segment 302 is dividedinto two subsegments, earlier subsegment 305 b and later subsegment 305a. The example formula is as follows:

${CrankshaftAngularAcceleation} = \frac{{{AvgSpeed}\left( {305a} \right)} - {{AvgSpeed}\left( {305b} \right)}}{\Delta \; {{Time}\left( {305{ab}} \right)}}$

where the AvgSpeed (305 a) and AvgSpeed (305 b) are the averagevelocities of the crankshaft over subsegments 305 a and 305 b,respectively, and Δ Time (305 ab) refers to the time needed for thecrankshaft to rotate from the midpoint of subsegment 305 b to themidpoint of subsegment 305 a. While the subsegments 305 a and 305 b areshown as having equal duration, this need not be the case. Also, thesubsegments 305 a and 305 b need not be continuous, i.e. there may be agap between the segments. The timing of the subsegments relative to thecrankshaft rotation may be adjusted depending on the engine operatingconditions and the misfire detection algorithm. In some cases more thantwo subsegments may be used. The subsegment durations and timing mayvary depending on the engine operating conditions. The average enginespeed may be determined by measuring the lapsed time between referencemarks on the crankshaft passing a fixed reference point. In variousembodiments, the crankshaft reference marks may be equally distributedaround the crankshaft at approximately 6 degree intervals. The rawsignal from crank angle may be processed to calculate the average speedin a subsegment, acceleration between subsegments, and the jerk (changein acceleration between pairs of subsegments). In various embodiments,measurement of jerk requires use of at least three subsegments, so thata change in acceleration may be measured. Higher order time derivativesof acceleration may also be used in misfire determination, with aconcomitant increase in the number of subsegments. Another examplemethod of calculating crank acceleration is to take a time derivative ofraw engine speed (RPM) signal and apply a bandpass filter such as Type 1Chebyshev filter. Various other filtering algorithms may be applied tothe crank signal to improve the accuracy of all these measurements.Generally, the calculation of engine parameter change is performed formultiple firing events for each working chamber. Thus, the engineparameter measurement module builds a history of firing events for eachworking chamber, as well as corresponding engine parameter changes(e.g., crankshaft angular acceleration data) for the working chamber.This data is later used to help determine whether a particular workingchamber is misfiring or not.

A variety of engine parameters may be measured in step 106. In someembodiments, as noted above, a crankshaft-related parameter, such as thecrankshaft angular acceleration or its derivative (jerk), may bemeasured. In other embodiments, the engine parameter measurementinvolves an analysis of exhaust gases. For example, as previouslydiscussed, various designs involve measuring a change in an amount ofoxygen in the exhaust of the engine over a corresponding window orperiod of time. This change is associated with a particular targetfiring opportunity of a target working chamber. Such changes can provideinsight into whether the target working chamber has misfired.

The misfire detection module 208 receives the firing information fromthe firing control unit 204 and/or the firing timing determinationmodule 202 and the above engine parameter measurement data from theengine parameter measurement module 206. Returning to FIG. 1, at step108, the misfire detection module 208 uses a pressure model to helpdetermine an expected change in the engine parameter during the window.This expected change will later be compared with the change measured instep 106 to determine whether a misfire has occurred.

The pressure model is used to model a pressure within each of theworking chambers of the engine during the window. In variousembodiments, the misfire detection module 208 uses the modeled pressureto estimate the torque generated by the working chamber, which in turncan be used to help predict a change in the engine parameter (e.g.,crankshaft acceleration) during the window. The modeling of pressurewithin multiple working chambers allows the misfire detection module 208to take into account the different operational states of each of theworking chambers, and their corresponding effects on the change in theengine parameter.

In skip fire engine control systems, the pressure within each cylindermay vary widely, depending in part on whether a particular workingchamber has or will be skipped or fired. The skip/fire decision for eachworking chamber is in the firing information that the misfire detectionmodule 208 receives from the firing control unit 204. The misfiredetection module 208 is arranged to use this firing information to modelthe pressure within one, some or all of the working chambers during thewindow. Any suitable pressure model may be used. A particularimplementation of a pressure model will be described in more detaillater in the application.

The skipping or firing of a working chamber can have a large impact onthe pressure dynamics within a working chamber during any given timeinterval. Consider the example of FIG. 3, in which during the window302, working chamber 8 is being fired. In this example, the workingchambers are fired or skipped in the order 1-8-7-2-6-5-4-3. The workingcycles of the other working chambers are out of phase with the workingcycle of working chamber 8 at consistent intervals. Accordingly, whileworking chamber 8 is in the first half of a power stroke, workingchamber 1 is in the second half of its power stroke during the sametarget window 302. If working chamber 1 is being fired, the combustionprocess will tend to substantially increase the pressure in the workingchamber. If working chamber 1 is skipped (e.g., using a low pressurespring), however, no combustion takes place and the pressure dynamicswithin the working chamber will be substantially different. The pressuremodel takes into account the different pressure effects of firing orskipping a working chamber.

Returning to FIG. 1, at step 110, based on the pressure model, themisfire detection module 208 determines an expected change in the engineparameter during the window. This step may be performed in any suitablemanner Various implementations, for example, involve modeling thepressure within each working chamber during a particular window andusing the modeled pressure to estimate a torque generated by eachworking chamber. The modeling of the torque generated by each workingchamber may be based on a wide variety of engine parameters that feedinto the pressure model. The engine parameters may include, but notlimited to, cam timing, engine speed, mass air charge, cylinder load andmanifold absolute pressure. In various embodiments, the torque generatedby all of the working chambers is summed and used to determine theexpected change in the engine parameter (e.g., crankshaft acceleration)during the window. Additionally any torque loads arising from vehicleaccessories, such as air conditioning, battery charging, etc., may beincluded in the overall torque model. Any suitable feature of the torquemodel and associated operations described in U.S. patent applicationSer. No. 14/207,109, which is incorporated by reference herein in itsentirety for all purposes, may also be used in method 100.

At step 112, the misfire detection module 208 determines whether amisfire occurred in the target working chamber during the target firingopportunity. In various embodiments, this determination involvescomparing the change in the engine parameter measured in step 106 withthe expected change in the engine parameter determined using themulti-cylinder pressure model (steps 108 and 110). Consider an examplein which the engine parameter is crankshaft acceleration. If a misfirehas taken place in the target working chamber, then combustion in thetarget working chamber was incomplete and the crankshaft angularacceleration should be reduced. In some embodiments, the misfiredetection module 208 thus determines whether the measured change incrankshaft acceleration falls below the expected change in thecrankshaft acceleration (i.e., as determined using the aforementionedmulti-cylinder pressure model) by a predetermined amount. If so, themisfire detection module determines that a misfire has (possibly) takenplace in the target working chamber. If not, the misfire detectionmodule determines that a misfire has not taken place. It should beappreciated that the above approach is simplified and exemplary and thata wide variety of methods may be used to make this misfiredetermination, some of which will be described later in thisapplication.

The use of the pressure model (step 108), the determination of theexpected change in the engine parameter (step 110) and the misfiredetermination (step 112) may be performed in a wide variety of ways. Oneapproach is illustrated in FIG. 4. FIG. 4 is a block diagramillustrating a technique for calculating an expected change incrankshaft angular acceleration using a cylinder pressure model. FIG. 4includes some of the modules illustrated in FIG. 2 i.e., the engineparameter measurement module 206 and the misfire detection module 208.In this example, the misfire detection module 208 includes varioussubmodules i.e., an indicated torque module 460, a predicted crankshaftacceleration module 480 and a misfire determination unit 490. It shouldalso be noted that input signal rationality checks may be performed ineach module, in order to determine integrity of input signals to ensurerobustness of the strategy.

In the illustrated embodiment, an analytical cylinder pressure modelbased on an ideal combustion stroke of an internal combustion engine isdeveloped to estimate cylinder torque under a variety of operatingconditions including the effects of skip fire operation (e.g., step 108of FIG. 1.) The model is applicable to many types of engine cycles, suchas Otto, Atkinson, Miller, Diesel, etc., using appropriate values thatcharacterize the combustion event. The model predicts pressure in eachcylinder not only during a fired cycle, but also during skipped cycles.The modeled cylinder pressure is then used to calculate the indicatedtorque based on a simple crank-slider mechanism.

In this example, a main concept of the analytical pressure model assumesthat the cylinder pressure p(θ) modeled as the interpolation between twoasymptotic pressure traces as illustrated in FIG. 5. The compressionstroke 512 is modeled by a polytropic process characterized by apolytropic exponent k_(c) and the thermodynamic state at intake valveclosing (IVC). The reference states at IVC are determined fromexperimental data. These traces determine the compression asymptote upuntil ignition. The expressions for pressure and temperature for thisprocess are

$\begin{matrix}{{p_{c}(\theta)} = {p_{IVC}\left( \frac{V_{IVC}}{V(\theta)} \right)}^{k_{c}}} & (1) \\{{T_{c}(\theta)} = {T_{IVC}\left( \frac{V_{IVC}}{V(\theta)} \right)}^{k_{c} - 1}} & (2)\end{matrix}$

The expansion asymptote 510 is also described by a polytropic processwith polytropic exponent k_(c). The quantities p₃, T₃, and V₃ correspondto point 3 in the ideal Otto cycle depicted in the pressure-volume (P-V)diagram shown in FIG. 6. The modeled combustion process moves betweenpoints 2 and 3 on the P-V diagram.

$\begin{matrix}{{p_{e}(\theta)} = {p_{3}\left( \frac{V_{3}}{V(\theta)} \right)}^{k_{e}}} & (3) \\{{T_{e}(\theta)} = {T_{3}\left( \frac{V_{3}}{V(\theta)} \right)}^{k_{e} - 1}} & (4)\end{matrix}$

The temperature rise ΔT_(comb) due to combustion is added to T₂ andstate (P₂,T₂) can be obtained from equation (1) and (2).

$\begin{matrix}{T_{3} = {T_{2} + {\Delta \; T_{comb}}}} & (5) \\{p_{3} = {p_{2}\frac{T_{3}}{T_{2}}}} & (6) \\{{\Delta \; T_{comb}} = \frac{m_{f}q_{HV}ɛ}{c_{v}m_{tot}}} & (7)\end{matrix}$

Where fuel mass m_(f), heating value q_(HV), conversion efficiency ε,specific heat c_(v), and total mass m_(tot) are used.

The interpolation between two asymptotes is the interpolated pressure514, based on the pressure ratio approach based on fitting heat releasewith the well-known Wiebe function described by parameters a, start ofcombustion angle θ_(SOC), combustion duration Δθ, and exponent m, whichcan be derived from experimental data. The pressure ratio is modeled by

$\begin{matrix}{{{PR}(\theta)} = {1 - {^{- {a{(\frac{\theta - \theta_{SOC}}{\Delta \; \theta})}}}}^{m + 1}}} & (8)\end{matrix}$

This can then be used for the interpolation

p(θ)=(1−PR(θ))·p _(c)(θ)+PR(θ)·pε(θ)  (9)

The procedure above provides a simple and complete model for pressurebetween IVC (intake valve closure) and EVO (exhaust valve opening). Thepressure during gas exchange may be set to the intake manifold pressure.However, for skipped cycles, the pressure will drop below intakemanifold pressure during the intake stroke. To properly model thepressure evolution during a skipped cycle, a polytropic processreferenced to either the BDC (bottom dead center) or the exhaust valveclosing (EVC) may be used. The pressure at EVC (exhaust valve closing)for firing cycles may be derived from experimental data.

$\begin{matrix}{{p(\theta)} = {p_{EVC}\left( \frac{V_{EVC}}{V(\theta)} \right)}^{k_{c}}} & (10)\end{matrix}$

A comparison of the modeled cylinder pressure 804 with the measuredcylinder pressure 802 is shown in FIG. 7. The pressures during skippedcycles as well as the firing cycles are captured well enough foraccurate torque prediction as described next. For clarity the initialsection of curve 804 (solid line) is omitted and the final section ofcurve 802 (dashed line) is omitted. Inspection of FIG. 7 shows that themodel works well in predicting the cylinder pressure.

The gas force acting on a piston connected to the crank shaft by a rodwith a crank slider mechanism produces at each instant an “indicatedtorque”

$\begin{matrix}{{T_{{cyl},i}(\theta)} = {\left( {{P_{{cyl},i}(\theta)} - P_{crank}} \right){Ar}\frac{\sin \left( {\theta + \beta} \right)}{\cos \; \beta}}} & (11) \\{\beta = {\sin^{- 1}\frac{\delta + {r\; {\sin \left( {\theta - \phi} \right)}}}{l}}} & (12) \\{\phi = {\sin^{- 1}\frac{\delta}{\left( {r + l} \right)}}} & (13)\end{matrix}$

where P_(crank), r, δ, l and A are crankcase pressure, crank radius, pinoffset, connecting rod length and piston face cross-section area,respectively. The resultant engine indicated torque is just sum of thecontributions from each cylinder.

T _(l)(θ)=Σ_(Namcyl) T _(cyl,i)(θ)  (14)

The indicated torque may be determined using the methods described abovein the Indicated Torque Module 460 (FIG. 4). Inputs to the IndicatedTorque Module 460 include engine operating conditions 462, hardwareparameters 464, combustion parameters 466, and a cylinder deactivationflag 468. Engine operating conditions 462 may include engine speed 472,intake manifold absolute pressure (MAP), intake manifold airtemperature, air per cylinder, cam phasing, and other variables.Hardware parameters 464 may include connecting rod length, compressionratio, valve opening window, pin offset and other design parameters.Combustion parameters 466 may include injection timing, spark timing,heat release characteristics during combustion and other parametersdescribing the combustion details. The cylinder deactivation flag 468describes whether a cylinder is being fired or skipped.

The indicated engine torque 474 obtained from using the cylinderpressure model by the Indicated Torque Module 460 may be used todetermine crank angular acceleration in the Predicted Crank AccelerationModule 480 (e.g., step 110 of FIG. 1.) The engine dynamic model used forthis derivation is

$\begin{matrix}{{{{J_{eq}\overset{¨}{\theta}} + {M_{eq}{r^{2}\left\lbrack {{{f_{1}(\theta)}\overset{¨}{\theta}} + {{f_{2}(\theta)}{\overset{.}{\theta}}^{2}}} \right\rbrack}{f_{3}(\theta)}}} = {{T_{i}(\theta)} - {T_{fp}(\theta)} - {T_{L}(\theta)}}}{where}} & (15) \\{{f_{1}(\theta)} = {{f_{3}(\theta)} = {{\sin \; \theta} + {\frac{r}{2l}\sin \; 2\theta}}}} & (16) \\{{f_{2}(\theta)} = {{\cos \; \theta} + {\frac{r}{l}\cos \; 2{\theta.}}}} & (17)\end{matrix}$

θ is the crank angle, {dot over (θ)} and {umlaut over (θ)} are theangular velocity and the angular acceleration of the crankshaft,respectively. l is the connecting rod length, and r is the crank radius.J_(eq) is the moment of inertia of the crankshaft, flywheel, gear androtating part of connecting rod, and M_(eq) is the mass of the piston,rings, pin and linear motion part of the connecting rod. T_(i)(θ),T_(fp)(θ), and T_(L)(θ) are the indicated engine torque 474, frictiontorque 482, and load torque 476, respectively. Other inputs 478, such asaccessory loads, may be considered in Eq. (15). The derivation of thisequation can be found in the literature, for example Zweiri, et. al.“Instantaneous friction components model for transient engineoperation”, Proc. Inst. Mech. Eng. Part J. Automob. Eng. Vol. 214, no. 7pp. 809-824, July 2000.

The crank angle θ, crank angular velocity {dot over (θ)}, and equivalentmass M_(eq) may be measured, and the indicated engine torque T_(i)(θ)474 is given by the model previously described. The friction torqueT_(fp)(θ) 482 may be determined by a lookup table obtained fromexperiments which relates the crank RPM to friction torque. The combinedmoment of inertia of crankshaft, flywheel, gear, and rotating part ofconnecting rod, J_(eq), may also be determined experimentally for eachgear. The load torque T_(L)(θ) 476 may be estimated from the differencebetween the engine speed and turbine shaft speed through equation (18)for the torque converter and torque converter clutch. T_(p) is thetorque converter torque and T_(tcc) is the torque converter clutchtorque. K_(i) is calculated by a lookup table obtained from experimentsand torque converter clutch gain K_(tcc) and a constant a may also bedetermined experimentally.

$\begin{matrix}{{T_{L} = {T_{p} + T_{tcc}}}{T_{p} = {\frac{\omega_{e} - \omega_{t}}{{\omega_{e} - \omega_{t}}}\frac{\omega_{e}^{2}}{K_{i}^{2}}}}{T_{tcc} = {K_{tcc}{\tanh \left( \frac{\omega_{e} - \omega_{t}}{\alpha} \right)}}}} & (18)\end{matrix}$

The variables ω_(e) and ω_(t) are the angular speed of crankshaft andturbine shaft, respectively. A discrete-time low-pass filters may beapplied to T_(p) and T_(tcc) to remove high frequency components. Thelow-pass filter may be given by the following transfer function

$\begin{matrix}{{F(z)} = \frac{b}{z - a}} & (19)\end{matrix}$

where a and b are filter constants.

Equation (15) may be solved for {umlaut over (θ)} using the measuredcrank angular velocity {dot over (θ)} via

$\begin{matrix}{\overset{¨}{\theta} = {\frac{1}{J_{eq} + {M_{eq}r^{2}f_{1}f_{3}}}\left\lbrack {{{- M_{eq}}r^{2}f_{1}f_{2}{\overset{.}{\theta}}^{2}} + {T_{i}(\theta)} - {T_{fp}(\theta)} - {T_{L}(\theta)}} \right\rbrack}} & (20)\end{matrix}$

The predicted crank acceleration 486 obtained by the PredictedCrankshaft Acceleration Module 480 using the model described above maybe compared with the measured crank angular acceleration 488 determinedby the Engine Parameter Measurement Module 206 (e.g., step 106 of FIG.1). Measured crank acceleration 488 may be computed from the measuredcrank angular speed in the 6-degree angle domain, where the crankangular speed is sampled at every 6 crank angle degrees, by thefollowing formulae to obtain the derivative and also reduce the effectof measurement noise.

$\begin{matrix}{{A_{time}(n)} = \frac{\frac{42}{\sum\limits_{k = 1}^{7}{T_{d}\left( {n - k} \right)}} - \frac{42}{\sum\limits_{k = 8}^{14}{T_{d}\left( {n - k} \right)}}}{\sum\limits_{k = 4}^{10}{T_{d}\left( {n - k} \right)}}} & (21) \\{{T_{d}(n)} = {\frac{6}{360 \cdot \frac{r(n)}{6}} = {\frac{1}{10}\frac{1}{r(n)}}}} & (22)\end{matrix}$

r(n) is the crank angular speed at time step n in 6 degree angle domain.The acceleration formulae (21) can also be approximated as the doubleaverage of acceleration shown below.

$\begin{matrix}\begin{matrix}{{A_{rpn}(n)} = \frac{{\frac{1}{7}{\sum\limits_{k = 1}^{7}{r\left( {n - k} \right)}}} - {\frac{1}{7}{\sum\limits_{k = 8}^{14}{r\left( {n - k} \right)}}}}{\sum\limits_{k = 4}^{10}{T_{d}\left( {n - k} \right)}}} \\{= {\frac{1}{7}{\sum\limits_{k = 1}^{7}{\frac{1}{7}{\sum\limits_{m = 0}^{6}{a\left( {n - k - m} \right)}}}}}}\end{matrix} & (23)\end{matrix}$

Here the term T_(d)(n) is treated as a constant during the time stepsconsidered and Euler's rule is used to derive the relationship betweenr(n) and the acceleration a(n). FIG. 8 plots the crank shaftacceleration versus time for an engine running under substantiallysteady-state conditions. FIG. 8 compares the two averaging methods, thatbased on Eq. (21), curve 810, and that based on Eq. (23), curve 812,applied to the same angular speed signal generated for validationpurposes. The peaks in the curves 810 and 812 correspond to cylinderfirings and the dips correspond to skipped firing opportunities. Forclarity the initial section of curve 812 (dashed line) is omitted andthe final section of curve 810 (solid line) is omitted. Inspection ofFIG. 8 illustrates that the difference between the two methods isnegligible. This comparison demonstrates that the acceleration obtainedfrom the engine dynamics model can be filtered in 6-degree domain andcan be compared with the measured acceleration obtained from Eq. (21).

A high-pass filter may then be applied to both measured and modeledaccelerations. The high-pass filter removes any mean value offset errorsthat may be present in the acceleration estimate, making it easier tocompare the characteristics of the measured and simulated accelerationsrelevant for high pressure exhaust spring detection. The high-passfilter may be given by

y(z)=−a ₁ y(n−1)−a ₀ y(n−2)+b ₂ x(n)+b ₁ x(n−)+b ₀ x(n−2).  (24)

where a₀, a₁, b₀, b₁, and b₂ are the appropriate filter coefficientsdetermined by experimental data. Based on the comparison of the measuredand modeled accelerations, the misfire detection module determineswhether a misfire took place (e.g., step 112 of FIG. 1.)

The misfire detection module may use a wide variety of techniques todetermine whether a misfire took place (step 112. In some embodiments,for example, the misfire determination is based at least in part on thefollowing formula:

$X = \frac{A - B}{A}$

where A is the expected change in the engine parameter (e.g., asdetermined in step 110) and B is the measured change in the engineparameter (e.g., as determined in step 106). If the value X exceeds aparticular predefined threshold, then the misfire determination moduledetermines that a misfire has (possibly) occurred. If the value X doesnot exceed the predefined threshold, then the misfire determinationmodule determines that a misfire has not occurred.

In some applications, the accuracy of the misfire determination processmay be improved by adjusting the above formula as follows:

$X = \left( {\frac{A - B}{A^{\prime}} + k} \right)^{z}$

where A′ is a low-pass filtered mean of A. A′ can also be an expectedchange in the engine parameter and may be directly proportional to massair charge per working chamber. Z is any suitable exponent e.g., in someapplications, Z=3 works well. k is a predefined threshold such that thevalue of X of 1 or greater indicates a possible misfire, and the valueof X of less than 1 indicates that a misfire has not occurred. In someimplementations, it has been determined that above adjustments can helpreduce the amount of error caused by the complexity of the pressuremodel and/or improve the accuracy of the misfire determination process.

Generally, when an instance of misfire is detected for a particularworking chamber in which the aforementioned misfire threshold isexceeded, the information is stored in an additional module not shown inFIG. 4 in which a misfire counter for that working chamber isincremented. Typically, multiple firing events are monitored in order toconfirm that a working chamber has misfired. In some embodiments, forexample, the above misfire determination techniques are executed formany, almost all, or all firing events. Generally, a firing eventinvolves an attempt to actually fire a working chamber, as opposed tothe skipping of the working chamber. Each firing event is associatedwith the identity of a particular working chamber and data indicatingwhether the misfire threshold had been exceeded. This information isused to build a database of firing events. Thus, the misfire detectionmodule 208 stores a history for each working chamber that indicates thenumber of firing events in which the misfire threshold was exceeded. Insome embodiments, if a particular working chamber is associated withmultiple firing events of which at least a predetermined percentage ornumber involves the exceeding of a misfire threshold, a determination ismade that the working chamber is misfiring and the appropriate errorsignal is communicated to the vehicle driver via the OBD system,typically a malfunction indicator lamp on the vehicle dashboard. Anappropriate error code may also be sent to the OBD interface forsubsequent diagnostic evaluation.

It should be appreciated that the misfire determination process is notlimited to the aforementioned formulas, and that any suitable techniquefor determining misfire may be used. In various embodiments, forexample, an adjusted version of the above formulas is used e.g., themisfire determination involves exponentiation of a value based on A, Band A′, but the value and the aforementioned variables or formulas maybe further adjusted in various ways not explicitly described above.

After a particular working chamber is determined to be misfiring asindicated above, additional steps may be taken. Such steps include butare not limited to the skipping of the misfiring working chamber, thedisplaying of an alert and the use of a fixed firing sequence. These andother steps are described in co-assigned U.S. patent application Ser.No. 14/207,109 (hereinafter referred to as the '109 application), whichis incorporated herein in its entirety for all purposes. Any of theabove techniques described in the '109 application may be performedafter the steps of method 100 of FIG. 1 are performed.

The arrangements and techniques described in FIGS. 1-4 can be used for avariety of applications. In various embodiments, the illustrated misfiredetection system 200 (FIG. 2) is stored in an engine control unit of avehicle and/or is part of an onboard diagnostics system. In otherembodiments, it may be stored in an external diagnostic device that isused to examine the performance of an engine. Any of the aforementionedmodules, systems and operations may be stored in the form of hardware,software or both.

Referring next to FIG. 9, an example method 900 for using an adaptivemodel will be described. As discussed above in connection with method100 of FIG. 1, for various applications, it is useful to use a model(e.g., a pressure or torque model) to estimate a change in an engineparameter (e.g., crankshaft acceleration) during a particular window.This estimate may be used to help determine whether any engine errorshave taken place. However, in some cases, possibly due to unforeseenconditions and the various assumptions that the model is based on, theestimate provided by the model may be inaccurate. For example, as anengine ages leakage past the piston rings and valve seats may impact thepressures and associated torque generated by a cylinder. To improve theaccuracy of the model, the engine parameter is measured during the samewindow and the model is adjusted based on the measurement. Method 900describes an example of this approach. The techniques described inmethod 900 may be applied to any of the aforementioned methods (e.g.,method 100 of FIG. 1) and may be performed in skip fire or conventionalengine control systems.

Initially, at step 402, firing information is obtained. At step 404, awindow is assigned to a target firing opportunity. At step 406, a changein an engine parameter (e.g., crankshaft acceleration) is measured. Atstep 408, a model is used to estimate an expected change in the engineparameter. The model may be any suitable model used to estimate a changein the engine parameter (e.g., a model that estimates pressure withinand/or torque generated by each working chamber as described in method100 of FIG. 1, FIG. 4, etc.) Steps 402, 404, 406 and 408 may beperformed in the same manner as steps 102, 104, 106 and 108 of FIG. 1,respectively.

At step 410, the model is adjusted based on a comparison between theexpected change in the engine parameter (step 408) and the measuredchange in the engine parameter (step 406). In various embodiments, thisadjustment is only performed when the difference between the expectedchange and the measured change exceeds a predefined threshold. Theadjustment may be performed in a variety of ways. In some embodiments,for example, when steps 406 and 408 are repeated at a later time, amultiplier is applied to the expected change determined using the model.The multiplier reduces the offset between the measured and expectedvalues. Put another way, when step 408 is repeated and the modelgenerates another estimate of an expected change in the engineparameter, the estimate is adjusted to bring it more in line with acorresponding measurement of the engine parameter.

An example of this approach is illustrated in FIGS. 10 and 11. FIG. 10is a graph illustrating crankshaft acceleration as a function of time inan example implementation of the aforementioned model. In theillustrated embodiment of FIG. 10, the model has not been adjusted basedon the measurements performed in step 406. The solid line curveindicates the crankshaft acceleration over time as estimated using themodel. The dotted line curve indicates measurements of the crankshaftacceleration over time. The graphs indicate a mismatch between thedotted line and solid line curves. That is, the crankshaft accelerationestimated by the model tends to significantly overshoot the measuredcrankshaft acceleration.

In FIG. 11, the model is adjusted as described in step 410 based on acomparison of the results illustrated in FIG. 10. In FIG. 11, thedifference between the expected and measured changes in the crankshaftacceleration are thus significantly reduced. That is, the solid linecurve (which represents the crankshaft acceleration estimated by themodel) more closely approximates the dotted line curve (which representsthe measured crankshaft acceleration.) Put another way, the offsetbetween the expected engine parameter change (e.g., from step 408)generated by the model and the measured engine parameter change (e.g.,from step 406) has been reduced. It should be appreciated that thisapproach does not cause a problem in detecting an engine error, such asa misfire event, which is marked in the graph. At that point in time,the measured crankshaft acceleration is much lower than what waspredicted by the model. This difference may be used to identify apossible misfire (e.g., as previously discussed in connection with steps108 and 110 of FIG. 1.)

The adjustment of the model may be performed in any suitable manner Aparticular embodiment is illustrated in FIG. 12. FIG. 12 is a flowdiagram illustrating how an adaptive model may be used to estimate achange in crankshaft acceleration during a particular window. Thediagram indicates various calculations that can be performed to generatethe estimate (e.g., step 408.) Section 1204 of the flow diagram comparesone or more past measurements of changes in crankshaft acceleration(e.g., step 406) and one or more past estimates of the changes in crankacceleration based on the model (e.g., step 408). At multiplier block1206, a suitable multiplier is applied to one or more Crank Accel modelinput parameters 1208 based on the comparison. The adjusted modeledinputs 1202 may be used in the model 1210 to yield an adjusted modeledcrank acceleration 1212. Filtered adjusted modeled crank acceleration1212 may be used as a feedback signal to section 1204 and used in amisfire detection algorithm, for example the algorithm depicted in FIG.9.

The described methods and arrangements may be integrated into anysuitable skip fire engine control system. It should be appreciated thatthe described misfire detection system 200 may include additionalcomponents, features or modules that are not show in FIG. 2. Forexample, the firing sequences generated by the firing timingdetermination module 202 may be based on a firing fraction. In someembodiments, the misfire detection system 200 includes a firing fractioncalculator that determines this firing fraction based on a desiredtorque. A wide variety of firing fraction calculators, firing timingdetermination modules, powertrain parameter adjusting modules, ECUs,engine controller and other modules are described in co-assigned U.S.Pat. Nos. 7,954,474; 7,886,715; 7,849,835; 7,577,511; 8,099,224;8,131,445; and 8,131,447; U.S. patent application Ser. Nos. 13/774,134;13/963,686; 13/953,615; 13/953,615; 13/886,107; 13/963,759; 13/963,819;13/961,701; 13/963,744; 13/843,567; 13/794,157; 13/842,234; 13/004,839,13/654,244 and 13/004,844; and U.S. Provisional Patent Application Nos.61/080,192; 61/104,222; and 61/640,646, each of which is incorporatedherein by reference in its entirety for all purposes. Various enginediagnostic and misfire detection techniques are described in U.S.Provisional Patent Application Nos. 61/799,180 and 61/002,762 and U.S.patent application Ser. No. 14/207,109, which are also incorporatedherein by reference in their entirety for all purposes. Any of thefeatures, modules and operations described in the above patent documentsmay be added to the illustrated misfire detection system 200. In variousalternative implementations, these functional blocks may be accomplishedalgorithmically using a microprocessor, ECU or other computation device,using analog or digital components, using programmable logic, usingcombinations of the foregoing and/or in any other suitable manner.

Any and all of the described components may be arranged to refresh theirdeterminations/calculations very rapidly. In some preferred embodiments,these determinations/calculations are refreshed on a firing opportunityby firing opportunity basis although that is not a requirement. In someembodiments, for example, the described engine parameter changemeasurements, the adaptive adjustment of a model (e.g., step 410 of FIG.4) and the misfire/engine error determinations are performed on a firingopportunity by firing opportunity basis. An advantage of firingopportunity by firing opportunity operation of the various components isthat it makes the controller very responsive to changed inputs and/orconditions. Although firing opportunity by firing opportunity operationis very effective, it should be appreciated that the various componentscan be refreshed more slowly while still providing good control (e.g.,the determinations/calculations may be performed every revolution of thecrankshaft, every one or more working cycles, etc.).

The invention has been described primarily in the context of detectingmisfire in the skip fire operation of 4-stroke piston engines suitablefor use in motor vehicles. However, it should be appreciated that thedescribed misfire detection approaches are very well suited for use in awide variety of internal combustion engines. These include engines forvirtually any type of vehicle—including cars, trucks, boats, aircraft,motorcycles, scooters, etc.; and virtually any other application thatinvolves the firing of working chambers and utilizes an internalcombustion engine. The various described approaches work with enginesthat operate under a wide variety of different thermodynamiccycles—including virtually any type of two stroke piston engines, dieselengines, Otto cycle engines, Dual cycle engines, Miller cycle engines,Atkinson cycle engines, Wankel engines and other types of rotaryengines, mixed cycle engines (such as dual Otto and diesel engines),hybrid engines, radial engines, etc. It is also believed that thedescribed approaches will work well with newly developed internalcombustion engines regardless of whether they operate utilizingcurrently known, or later developed thermodynamic cycles.

In some embodiments, the firing timing determination module utilizessigma delta conversion to generate a skip fire firing sequence. Althoughit is believed that sigma delta converters are very well suited for usein this application, it should be appreciated that the modules mayemploy a wide variety of modulation schemes. For example, pulse widthmodulation, pulse height modulation, code division multiple access(CDMA) oriented modulation or other modulation schemes may be used todeliver the drive pulse signal. Some of the described embodimentsutilize first order converters. However, in other embodiments higherorder converters may be used. In still other embodiments, as describedin some of the aforementioned patent documents, a firing sequence isselected from a library of predefined firing sequences.

It should be also appreciated that any of the operations describedherein may be stored in a suitable computer readable medium in the formof executable computer code. The operations are carried out when aprocessor executes the computer code. Such operations include but arenot limited to any and all operations performed by the firing timingdetermination module 202, the firing control unit 204, the engineparameter measurement module 206, the misfire detection module 208, themisfire detection system 200, or any other module, component orcontroller described in this application.

The described embodiments work well with skip fire engine operation. Insome implementations, working chambers are fired under close to optimalconditions. That is, the throttle may be kept substantially open and/orheld at a substantially fixed positioned and the desired torque outputis met by varying the firing frequency. In some embodiments, during thefiring of working chambers the throttle is positioned to maintain amanifold absolute pressure greater than 70, 80, 90 or 95 kPa.

In some embodiments, the above techniques make use of the actual firinghistory of the cylinders so that only fired cylinders are actuallyconsidered by the misfire detection system. That is, when a cylinder isskipped, no effort is made to detect a misfire event with respect tothat specific cylinder (e.g., the method of FIG. 1 is applied such thatthe target firing opportunity always involves a target working chamberthat was arranged to be fired during the assigned window and notskipped.) In this way, the lack of the acceleration peaks during thetimeslots associated with the missed firing opportunities will not beinterpreted as misfires of the associated cylinders.

Various embodiments of the invention have been primarily described inthe context of a skip fire control arrangement in which cylinders aredeactivated during skipped working cycles by deactivating both theintake and exhaust valves in order to prevent air from being pumpedthrough the cylinders during skipped working cycles. However, it shouldbe appreciated that some skip fire valve actuation schemes contemplatedeactivating only exhaust valves, or only the intake valves toeffectively deactivate the cylinders and prevent the pumping of airthrough the cylinders. Several of the described approaches work equallywell in such applications. Further, although it is generally preferableto deactivate cylinders, and thereby prevent the passing of air throughthe deactivated cylinders during skipped working cycles, there are somespecific times when it may be desirable to pass air through a cylinderduring a selected skipped working cycle. By way of example, this may bedesirable when engine braking is desired and/or for specific emissionsequipment related diagnostic or operational requirements. The describedvalve control approaches work equally well in such applications.

Various implementations of the invention are very well suited for use inconjunction with dynamic skip fire operation in which an accumulator orother mechanism tracks the portion of a firing that has been requested,but not delivered, or that has been delivered, but not requested suchthat firing decisions may be made on a firing opportunity by firingopportunity basis. However the described techniques are equally wellsuited for use in virtually any skip fire application (operational modesin which individual cylinders are sometimes fired and sometime skippedduring operation in a particular operational mode) including skip fireoperation using fixed firing patterns or firing sequences as may occurwhen using rolling cylinder deactivation and/or various other skip firetechniques. Similar techniques may also be used in variable strokeengine control in which the number of strokes in each working cycle arealtered to effectively vary the displacement of an engine.

Although only a few embodiments of the invention have been described indetail, it should be appreciated that the invention may be implementedin many other forms without departing from the spirit or scope of theinvention. FIGS. 1 and 4, for example, illustrate a number of steps in amethod for detecting misfire or other engine errors. It should beappreciated that these operations need not take place in the illustratedorder, and one or more steps may be modified, reordered, removed orreplaced. There are also several references to the term, “cylinder.” Itshould be understood that the term cylinder should be understood asbroadly encompassing any suitable type of working chamber. Therefore,the present embodiments should be considered illustrative and notrestrictive and the invention is not to be limited to the details givenherein.

What is claimed is:
 1. A method for detecting misfire in an engine, theengine having a plurality of working chambers and being operated in askip fire manner, the method comprising: assigning a window to a targetfiring opportunity; attempting to fire a target working chamber duringthe target firing opportunity; measuring a change in an engine parameterduring the target firing opportunity; using a multi-cylinder pressuremodel to help determine an expected change in the engine parameterduring the target firing opportunity wherein the pressure model involvesestimating pressure in a skipped working chamber; and based on acomparison between the expected change and the measured change,determining whether the target working chamber misfired.
 2. A method asrecited in claim 1 wherein the engine parameter is crankshaftacceleration.
 3. A method as recited in claim 1 wherein themulti-cylinder pressure model involves modeling pressure within eachworking chamber during a time period between intake valve closure andexhaust valve opening of the target firing opportunity.
 4. A method asrecited in claim 1 wherein the pressure model takes into account atleast one selected from the group consisting of a rise of temperature ina working chamber due to combustion, fuel mass used to fuel combustion,energy conversion efficiency, ignition timing, residual fraction,leakage rate, fuel properties such as heating value, and total mass ofmixture in a working chamber.
 5. A method as recited in claim 1 furthercomprising using the pressure model to determine an expected torquegenerated by the working chambers of the engine during the target firingopportunity.
 6. A method as recited in claim 5 wherein the determinationof the expected torque is based on at least one selected from the groupconsisting of crankcase pressure, crank radius, pin offset, connectingrod length and piston face cross-sectional area.
 7. A method as recitedin claim 5 wherein: the expected torque, which is determined based onthe pressure model, is used to determine an expected crankshaftacceleration; and the determination of the expected crankshaftacceleration is based on at least one selected from the group consistingof engine speed, intake manifold absolute pressure, intake manifold airtemperature, air per cylinder, cam phasing, connecting rod length,compression ratio, valve opening window, pin offset, injection timing,spark timing, the moment of inertia of a crankshaft, the moment ofinertia of a flywheel, the moment of inertia of a rotating component, amass of a piston, a mass of a ring and a mass of a linear motioncomponent.
 8. A method as recited in claim 1 wherein the determinationof the misfire is based at least in part on A, B and A′ wherein A is theexpected change in the engine parameter based on the model, A′ is one ofan expected change in the engine parameter based on mass air charge anda low-pass filtered mean of A and B is the measured change in the engineparameter.
 9. A method as recited in claim 8 wherein the determinationof the misfire is based at least in part on $\frac{A - B}{A^{\prime}}.$10. A method as recited in claim 8 wherein the determination of themisfire further includes: calculating a value Y that is based on A, Band A′; and calculating a value X that is based on an exponentiation ofY; determining whether a misfire has occurred in the target workingchamber based at least in part on the value X.
 11. A method as recitedin claim 1 further comprising: detecting a first offset between theestimated expected change and the measured change in the engineparameter; adjusting the pressure model based on the first offset; andrepeating the pressure model usage and measurement operations, therebyproviding a second expected change and a second measured change in theengine parameter wherein a second offset between the second expectedchange and the second measured change is reduced relative to the firstoffset as a result of the model adjustment.
 12. A misfire detectionsystem for determining whether a particular working chamber in an enginehas misfired, the engine being operated in a skip fire manner, themisfire detection system comprising: an engine parameter measurementmodule that is arranged to: assign a window to a target firingopportunity; and measure a change in an engine parameter during thetarget firing opportunity; and a misfire detection module that isarranged to: use a pressure model to help determine an expected changein the engine parameter during the target firing opportunity wherein thepressure model involves estimating pressure in a skipped workingchamber; and determine whether the target working chamber misfired basedon a comparison between the expected change and the measured change. 13.A misfire detection system as recited in claim 12 wherein the engineparameter is crankshaft acceleration.
 14. A misfire detection system asrecited in claim 12 wherein the pressure model involves modelingpressure within a working chamber during a time period between intakevalve closure and exhaust valve opening.
 15. A misfire detection systemas recited in claim 12 wherein the multi-cylinder pressure model takesinto account at least one selected from the group consisting of a riseof temperature in a working chamber due to combustion, fuel mass used tofuel combustion, energy conversion efficiency, ignition timing, residualfraction, leakage rate, fuel properties such as heating value, and totalmass of mixture in a working chamber.
 16. A misfire detection system asrecited in claim 12 wherein the misfire detection module is furtherarranged to determine an expected torque generated by the workingchambers of the engine during the firing opportunity.
 17. A misfiredetection system as recited in claim 16 wherein the determination of theexpected torque using the pressure model is based on at least oneselected from the group consisting of crankcase pressure, crank radius,pin offset, connection rod length and piston face cross-sectional area.18. A misfire detection system as recited in claim 16 wherein: theexpected torque, which is determined based on the pressure model, isused to determine an expected crankshaft acceleration; and thedetermination of the expected crankshaft acceleration is based on atleast one selected from the group consisting of engine speed, intakemanifold absolute pressure, intake manifold air temperature, air percylinder, cam phasing, connecting rod length, compression ratio, valveopening window, pin offset, injection timing, spark timing, the momentof inertia of a crankshaft, the moment of inertia of a flywheel, themoment of inertia of a rotating component, a mass of a piston, a mass ofa ring and a mass of a linear motion component.
 19. A misfire detectionsystem as recited in claim 12 wherein the misfire determination is basedat least in part on A, B and A′ wherein A is the expected change in theengine parameter based on the model, A′ is one of a low-pass filteredmean of A and an expected change in the engine parameter based on massair charge, and B is the measured change in the engine parameter.
 20. Amisfire detection system as recited in claim 19 wherein thedetermination of the misfire is based at least in part on$\frac{A - B}{A^{\prime}}.$
 21. A misfire detection system as recited inclaim 19 wherein the misfire detection module is further arranged to:calculate a value Y that is based on A, B and A′; and calculate a valueX that is based on an exponentiation of Y; determine whether a misfirehas occurred in the target working chamber based on the value X.
 22. Amisfire detection system as recited in claim 12 wherein the misfiredetection module is further arranged to: detect a first offset betweenthe estimated expected change and the measured change in the engineparameter; adjust the pressure model based on the first offset; andrepeat the pressure model usage and measurement operations, therebyproviding a second expected change and a second measured change in theengine parameter wherein a second offset between the second expectedchange and the second measured change is reduced relative to the firstoffset as a result of the model adjustment.
 23. A method for using anadaptive model to estimate a change in an engine parameter, the methodcomprising: assigning a window to a first target firing opportunity;attempting to fire a target working chamber during the first targetfiring opportunity; measuring a first change in an engine parameterduring the first target firing opportunity; using a model to helpestimate a first expected change in the engine parameter during thefirst target firing opportunity wherein there is an offset between thefirst expected change and the first measured change; and based on themeasured first change in the engine parameter, adjusting the model tohelp reduce the offset.
 24. A method as recited in claim 23 wherein themodel involves modeling an engine characteristic for a skipped workingchamber.
 25. A method as recited in claim 23 wherein the engineparameter is crankshaft acceleration.
 26. A method as recited in claim23 wherein the model is one or more selected from the group consistingof a pressure model and a torque model.
 27. A method as recited in claim23 wherein the adjustment of the model further comprises: determining amultiplier based on the first expected change and the first measuredchange in the engine parameter; and applying the multiplier to one ormore input parameters to the model that estimates a second expectedchange in the engine parameter.
 28. A method as recited in claim 23further comprising: assigning a window to a second target firingopportunity; attempting to fire a target working chamber during thesecond target firing opportunity; measuring a second change in an engineparameter during the second target firing opportunity; using the modelto help estimate a second expected change in the engine parameter duringthe second target firing opportunity wherein there is a second offsetbetween the second expected change and the second measured change thatis lower than the first offset as a result of the model adjustment. 29.A method as recited in claim 23 further comprising: using the adjustedmodel to estimate a second expected change in the engine parameterduring a second target firing opportunity; and measuring a second changein the engine parameter during the second target firing opportunity;based on a comparison of the second expected change and the secondmeasured change in the engine parameter, determining whether a misfirehas occurred in the target working chamber.